Control system and method for improved efficiency of particulate matter filter regeneration

ABSTRACT

An engine control system includes an injection determination module and an injection distribution module. The injection determination module determines a desired amount of hydrocarbons (HC) to inject into exhaust gas produced by an engine for regeneration of a particulate matter (PM) filter. The injection distribution module controls a ratio of auxiliary injection to post-combustion injection based on engine load, engine speed, and the desired amount of HC injection, wherein auxiliary injection includes injecting HC into the exhaust gas, and wherein post-combustion injection includes injecting HC into cylinders of the engine during a period after combustion.

FIELD

The present disclosure relates to internal combustion engines, and moreparticularly to a system and method for controlling hydrocarbon (HC)injection into exhaust gas produced by an engine to improve efficiencyof particulate matter (PM) filter regeneration.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Internal combustion engines combine air and fuel to create an air/fuel(A/F) mixture that is combusted within a plurality of cylinders. Thecombustion of the A/F mixture drives pistons which rotatably turn acrankshaft generating drive torque. Specifically, air may be drawn intothe cylinders and compressed using the pistons. Fuel may then becombined with (i.e., injected into) the compressed air causing thepressurized A/F mixture to combust (e.g., a compression ignition, or CIengine). For example, CI engines include diesel engines.

Alternatively, the air may be mixed with fuel to create the A/F mixtureprior to compression. The A/F mixture may then be compressed until theA/F mixture reaches a critical pressure and/or temperature andautomatically ignites (e.g., a homogeneous charge compression ignition,or HCCI engine). HCCI engines, however, may also “assist” ignition ofthe A/F mixture using spark from spark plugs. In other words, HCCIengines may ignite the A/F mixture using spark assist depending onengine operating conditions. For example, HCCI engines may use sparkassist at low engine loads.

Exhaust gas produced during combustion may be expelled from thecylinders into an exhaust manifold. The exhaust gas may include carbonmonoxide (CO) and hydrocarbons (HC). The exhaust gas may also includenitrogen oxides (NOx) due to the higher combustion temperatures of CIengines and HCCI engines compared to spark ignition (SI) engines. Anexhaust treatment system may treat the exhaust gas to remove CO, HC,and/or NOx. For example, the exhaust treatment system may include, butis not limited to, at least one of an oxidation catalyst (OC), aparticulate matter (PM) filter, a selective catalytic reduction (SCR)system, NOx absorbers/adsorbers, and catalytic converters.

SUMMARY

An engine control system includes an injection determination module andan injection distribution module. The injection determination moduledetermines a desired amount of hydrocarbons (HC) to inject into exhaustgas produced by an engine for regeneration of a particulate matter (PM)filter. The injection distribution module controls a ratio of auxiliaryinjection to post-combustion injection based on engine load, enginespeed, and the desired amount of HC injection, wherein auxiliaryinjection includes injecting HC into the exhaust gas, and whereinpost-combustion injection includes injecting HC into cylinders of theengine during a period after combustion.

A method includes determining a desired amount of hydrocarbons (HC) toinject into exhaust gas produced by an engine for regeneration of aparticulate matter (PM) filter, and controlling a ratio of auxiliaryinjection to post-combustion injection based on engine load, enginespeed, and the desired amount of HC injection, wherein auxiliaryinjection includes injecting HC into the exhaust gas, and whereinpost-combustion injection includes injecting HC into cylinders of theengine during a period after combustion.

In still other features, the systems and methods described above areimplemented by a computer program executed by one or more processors.The computer program can reside on a tangible computer readable mediumsuch as but not limited to memory, nonvolatile data storage, and/orother suitable tangible storage mediums.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples areintended for purposes of illustration only and are not intended to limitthe scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of an exemplary engine systemaccording to the present disclosure;

FIG. 2 is a functional block diagram of an exemplary control moduleaccording to the present disclosure;

FIG. 3 is a functional block diagram of an exemplary injectiondistribution module according to the present disclosure;

FIG. 4 is a graph illustrating exemplary exhaust gas temperature (EGT)control during particulate matter (PM) filter regeneration according tothe present disclosure; and

FIG. 5 is a flow diagram of an exemplary method for controllinginjection of hydrocarbons (HC) into engine exhaust gas during PM filterregeneration according to the present disclosure.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is in no wayintended to limit the disclosure, its application, or uses. For purposesof clarity, the same reference numbers will be used in the drawings toidentify similar elements. As used herein, the phrase at least one of A,B, and C should be construed to mean a logical (A or B or C), using anon-exclusive logical or. It should be understood that steps within amethod may be executed in different order without altering theprinciples of the present disclosure.

As used herein, the term module refers to an Application SpecificIntegrated Circuit (ASIC), an electronic circuit, a processor (shared,dedicated, or group) and memory that execute one or more software orfirmware programs, a combinational logic circuit, and/or other suitablecomponents that provide the described functionality.

Compression ignition (CI) (e.g., diesel) and homogeneous chargecompression ignition (HCCI) engines may include similar exhausttreatment systems. More specifically, exhaust treatment systems for CIand HCCI engines may include an oxidation catalyst (OC) located before(i.e., upstream from) a selective catalytic reduction (SCR) catalyst anda particulate matter (PM) filter. The OC oxidizes carbon monoxide (CO)and hydrocarbons (HC) to form carbon dioxide (CO₂) and water (H₂O). TheSCR catalyst (in conjunction with a dosing agent, such as urea) removesnitrogen oxides (NOx) from the exhaust gas.

The PM filter, on the other hand, removes PM from the exhaust gas. ThePM filter may remove PM from the exhaust gas until the PM filter issaturated. In other words, the saturation condition may correspond towhen the PM filter is full of PM (e.g., soot), after which aregeneration cycle may begin. The regeneration cycle may introduce HCinto the exhaust gas. The HC in the exhaust gas may be catalyzed by theOC in an exothermic reaction that generates heat and increases exhaustgas temperature (EGT). The increased EGT at the outlet of the OC (i.e.,at the inlet of the PM) may burn and/or breakdown the PM trapped in thePM filter, thus “regenerating” the PM filter.

Exhaust treatment systems, therefore, may further include one or more HCinjectors that inject HC (e.g., fuel) upstream from an OC in an exhauststream. This may be referred to as “auxiliary injection.” Alternatively,exhaust treatment systems may introduce HO into the exhaust gas byperforming post-combustion injection using fuel injectors of the engine.Conventional control systems control injection of HC into the exhauststream during PM filter regeneration based on exhaust flow.

Auxiliary injection and post-combustion injection, however, both havedisadvantages. More specifically, auxiliary injection may cause fuel topool on exhaust pipe walls, resulting in a poor mixture of the fuel withthe exhaust gas. The poor mixture may be difficult to catalyze by theOC, which may damage the OC and/or may result in decreased PM filterregeneration temperatures and increased emissions. Post-combustioninjection, on the other hand, may cause fuel to pool on cylinder wallsat low cylinder pressures (e.g., piston near bottom center). The poolingof fuel on the cylinder walls may dilute oil that lubricates the partswithin the cylinder, thus requiring more frequent oil changes and/ordecreased engine durability.

Accordingly, a system and method are presented that control a ratio ofauxiliary injection to post-combustion injection during PM filterregeneration based on engine operating parameters. In other words, thesystem and method may achieve the advantages of both auxiliary injectionand post-combustion injection while avoiding the correspondingdisadvantages (discussed above). Therefore, the system and method mayachieve more efficient regeneration of a PM filter, thereby protectingexhaust treatment system components (e.g., the OC, the PM filter, etc.)and/or decreasing emissions. Moreover, the system and method may commandpost-combustion injection in one bank of cylinders of the engine (e.g.,half), thereby allowing for exhaust gas recirculation (EGR) to furtherimprove the efficiency of the regeneration of the PM filter.

More specifically, the system and method may determine a desired amountof HC injection (i.e., a total quantity of HC injection) based on atemperature measured at an inlet of the PM filter. The system and methodmay also determine the desired amount of HC injection based on exhaustgas flow and a speed of the vehicle. The system and method may thencontrol the ratio of auxiliary injection to post-combustion injectionbased on engine load and engine speed. In other words, the system andmethod may determine amounts of auxiliary injection and post-combustioninjection. For example, the system and method may performproportional-integral-derivative (PID) control of the ratio and/or thedesired amount of HC injection based on the temperature measured at theinlet of the PM filter.

Referring now to FIG. 1, an engine system 10 includes a CI engine 12.For example only, the engine 12 may be a diesel engine or an HCCIengine. The engine 12 combusts an air/fuel (A/F) mixture to producedrive torque. Air is drawn into an intake manifold 14 through an inlet16. A throttle (not shown) may be included to regulate air flow into theintake manifold 14. Air within the intake manifold 14 is distributedinto a plurality of cylinders 20. While six cylinders are shown, it canbe appreciated that the engine 12 may include other numbers ofcylinders. A MAF sensor 18 may measure a rate of airflow through theinlet 16. For example, the airflow rate measurement may be used todetermine an engine load, which may correspond to a total amount of fuelrequired for combustion.

Fuel injectors 22 correspond to the cylinders 20. The fuel injectors 22may inject fuel directly into the cylinders 20 (i.e., direct fuelinjection). Alternatively, however, the fuel injectors 22 may injectfuel via intake ports of the cylinders 20 (i.e., port fuel injection). Apiston (not shown) compresses and combusts the A/F mixture within thecylinder 20. The piston drives an engine crankshaft (not shown) during apower stroke to produce drive torque. In one embodiment, the cylinders20 may include spark plugs (not shown) (e.g., for spark assist in anHCCI engine). The fuel injectors 22 may also inject fuel into thecylinders 20 after combustion of the A/F mixture (i.e., post-combustioninjection) to introduce hydrocarbons (HC) into exhaust gas.

The crankshaft (not shown) rotates at engine speed or a rate that isproportional to engine speed. A crankshaft speed (CS) sensor 24 measuresa rotational speed of the crankshaft. For example only, the CS sensor 24may be a variable reluctance sensor. Drive torque from the enginecrankshaft may be transferred to a driveline of a vehicle (e.g., wheels)via a transmission (not shown). A transmission output shaft speed (TOSS)sensor 26 measures a rotational speed of the output shaft of atransmission (not shown). In other words, the measurement from the TOSSsensor 26 may indicate vehicle speed. Both engine speed and vehiclespeed, however, may be measured or calculated using other suitablesensors and/or methods.

The exhaust gas resulting from the combustion within the cylinders 20 isexpelled into an exhaust manifold 28. An exhaust mass air flow (EMAF)sensor 30 generates an EMAF signal that indicates a rate of air flowingthrough the EMAF sensor 30. For example, the EMAF signal may indicate orbe used to determine exhaust flow through an exhaust treatment system32. Thus, the EMAF sensor 30 may be located between the exhaust manifold28 and the exhaust treatment system 32.

The exhaust treatment system 32 may treat the exhaust gas. The exhausttreatment system 32 may include an auxiliary HC injector 34, an OC 36,and a PM filter 40. The auxiliary HC injector 34 selectively injects HC(e.g., fuel) into the exhaust stream. As previously described, however,the fuel injectors 22 may perform post-combustion injection to introduceHC into the exhaust gas. The OC 36 oxidizes CO and HC in the exhaustgas. The PM filter 40 removes PM from the exhaust gas.

The exhaust treatment system 32 also includes temperature sensors 37 and39. Temperature sensor 37 may measure a temperature (T_(out)) of theexhaust gas at an outlet of the OC 36, and thus may be referred to as an“outlet temperature sensor.” Temperature sensor 39, on the other hand,may measure a temperature (T_(in)) at an inlet of the PM filter 40, andthus may be referred to as an “inlet temperature sensor.” The exhausttreatment system 32 may further include other temperature sensors (notshown) and/or NOx sensors (not shown) that measure exhaust gastemperature (EGT) and/or exhaust gas NOx concentration, respectively.

A control module 50 communicates with and/or controls various componentsof the engine system 10. The control module 50 may receive signals fromthe MAF sensor 18, CS sensor 24, the TOSS sensor 26, the EMAF sensor 30,the outlet temperature sensor 37, and the inlet temperature sensor 39.The control module 50 may also communicate with the PM filter 40 todetermine when a regeneration cycle is required. Alternatively, thecontrol module 50 may determine that regeneration of the PM filter 40 isrequired based on other parameters and/or modeling. For example, thecontrol module 50 may determine that regeneration of the PM filter 40 isrequired when exhaust flow is less than a predetermined exhaust flowthreshold (i.e., the PM filter 40 is restricted by PM).

The control module 50 may control a throttle (not shown), the fuelinjectors 22, the auxiliary HC injector 34, and an exhaust gasrecirculation (EGR) valve 46 (discussed in more detail below). Thecontrol module 50 may also implement the system and method of thepresent disclosure to improve efficiency of regeneration of the PMfilter 40. More specifically, the control module 50 may actuate the fuelinjectors 22 (i.e., post-combustion injection) and/or the auxiliary HCinjector 34 to control EGT and control regeneration of the PM filter 40.In one embodiment, the control module 50 may actuate fuel injectors 22corresponding to one bank of the cylinders 20 (e.g., half) to allow forEGR during regeneration of the PM filter 40.

The engine system 10 may further include an EGR system 42. The EGRsystem 42 includes the EGR valve 46 and an EGR line 44. The EGR system42 may introduce a portion of exhaust gas from the exhaust manifold 28into the intake manifold 14. The EGR valve 46 may be mounted on theintake manifold 14. The EGR line 44 may extend from the exhaust manifold28 to the EGR valve 46, providing communication between the exhaustmanifold 28 and the EGR valve 46. As previously described, the controlmodule 50 may actuate the EGR valve 46 to increase or decrease an amountof exhaust gas introduced into the intake manifold 14.

The engine 12 may also include a turbocharger 48. The turbocharger 48may be driven by the exhaust gas received through a turbine inlet. Forexample only, the turbocharger 48 may include a variable nozzle turbine.The turbocharger 48 increases airflow into the intake manifold 14 tocause an increase in intake MAP (i.e., manifold absolute pressure, orboost pressure). The control module 50 may actuate the turbocharger 48to selectively restrict the flow of the exhaust gas, thereby controllingthe boost pressure.

Referring now to FIG. 2, the control module 50 is shown in more detail.The control module 50 may include an injection determination module 60,an injection distribution module 70, and a regeneration control module80. The injection determination module 60 receives an exhaust gas flowand a vehicle speed. For example, the exhaust gas flow may be measuredby the EMAF sensor 30 and the vehicle speed may be measured by the TOSSsensor 26. The exhaust gas flow and the vehicle speed, however, may bemeasured using other sensors or modeled based on engine operatingparameters. The injection determination module 60 also receivestemperature T_(in) (measured by inlet temperature sensor 39).

The injection determination module 60 determines a desired amount of HCinjection (i.e., a total quantity of HC injection) based on temperatureT_(in). The injection determination module 60 may also determine thedesired amount of HC injection based on at least one of the exhaust gasflow and the vehicle speed. Moreover, in one embodiment the injectiondetermination module 60 performs PID control of the desired amount of HCinjection based on temperature feedback (i.e., T_(in)). For example, theinjection determination module 60 may include a lookup table thatincludes a plurality of desired amounts of HC injection relating tovarious inlet temperatures of the PM filter 40, exhaust gas flows,and/or vehicle speeds. For example only, the desired amount of HCinjection may decrease as exhaust gas flow increases. Alternatively, forexample only, the desired amount of HC injection may increase whentemperature T_(in) does not increase or increases too slowly.

The injection distribution module 70 receives the desired amount of HCinjection. The injection distribution module 70 also receives enginespeed and engine load. For example, the engine speed may be measuredusing the CS sensor 22 and the engine load may be measured using the MAFsensor 18. The engine speed and the engine load, however, may bemeasured using other sensors or modeled based on engine operatingparameters. The injection distribution module 70 determines an amount ofauxiliary injection and an amount of post-combustion injection based onthe desired amount of HC injection, the engine speed, and the engineload. More specifically, the injection distribution module 70 maydetermine a ratio of auxiliary injection to post-combustion injection.The injection distribution module 70 may then determine the amounts ofauxiliary injection and post-combustion injection based on the desiredamount of HC injection and the determined ratio. For example, theinjection distribution module 70 may include a lookup table thatincludes a plurality of ratios relating to engine speed and engine load.

The regeneration control module 80 receives the determined amounts ofauxiliary injection and post-combustion injection (AUX and PC,respectively). The regeneration control module 80 controls the auxiliaryHC injector 34 and fuel injectors 22 based on the determined amounts ofauxiliary injection and post-combustion injection, respectively.Alternatively, however, in one embodiment the injection distributionmodule 70 may determine rates of auxiliary injection and post-combustioninjection and the regeneration control module 80 may control theauxiliary injector 34 and the fuel injectors 22 according to thedetermines rates, respectively.

The regeneration control module 80 may control auxiliary injection andpost-combustion injection by generating control signals for theauxiliary HC injector 34 and the fuel injectors 22, respectively. Aspreviously described, however, in one embodiment the regenerationcontrol module 80 may generate control signals for fuel injectors 22associated with one bank of cylinders 20 to allow for EGR duringregeneration of the PM filter 40. While one regeneration control module80 is shown, two separate modules may be implemented to controlauxiliary injection and post-combustion injection, respectively.

Referring now to FIG. 3, an exemplary implementation of the injectiondistribution module 70 is shown in more detail. The injectiondistribution module 70 may include a ratio determination module 90. Theratio determination module 90 receives the engine load and the enginespeed. The ratio determination module 90 determines the ratio ofauxiliary injection to post-combustion injection. For example, the ratiodetermination module 90 may generate a factor (e.g., between 0 and 1).The injection distribution module 70 may multiply the desired amount ofHC injection by the factor to determine the amount of post-combustioninjection. The injection distribution module 80 may then subtract thedetermined amount of post-combustion injection from the desired amountof HC injection to determine the amount of auxiliary injection.

Referring now to FIG. 4, a graph is shown illustrating EGT control for avehicle traveling at 75 miles per hour (mph) according to the system andmethod of the present disclosure. More specifically, the graphillustrates EGT control during regeneration of a PM filter using 50%auxiliary injection and 50% post-combustion injection (i.e., a 1:1ratio). As shown, temperature T_(in) at the inlet of the PM filter 40accurately tracks the desired temperature steps (e.g., 565° C., 585° C.,etc.) throughout the regeneration process. The accurate tracking of thedesired temperature steps may result in improved efficiency during PMfilter regeneration.

Referring now to FIG. 5, a method for controlling injection of HC intoexhaust gas produced by the engine 12 begins in at 100. At 100, thecontrol module 50 determines whether the engine 12 is on. If true,control may proceed to 104. If false, control may return to 100. At 104,the control module 50 may determine whether regeneration of the PMfilter 40 is required. For example, regeneration of the PM filter 40 maybe required when exhaust flow is less than a predetermined threshold. Iftrue, control may proceed to 108. If false, control may return to 104.

At 108, the control module 50 may determine the desired amount of HCinjection. At 112, the control module 50 may determine the ratio ofauxiliary (AUX) injection to post-combustion (PC) injection. Forexample, the control module 50 may determine a factor using a lookuptable that includes a plurality of factors relating to engine load andengine speed

At 116, the control module 50 may determine the amounts of auxiliary(AUX) injection and post-combustion (PC) injection based on the desiredamount of HC injection and the determined ratio (or factor). Forexample, the control module 50 may determine the amount of auxiliary(AUX) injection based on a product of the factor and the desired amountof HC injection and may determine the amount of post-combustioninjection based on a difference between the desired amount of HCinjection and the determined amount of auxiliary injection

At 120, the control module 50 may control HC injection according to thedetermined amounts of auxiliary injection and post-combustion injection.At 124, the control module 50 may measure the temperature T_(in) at theinlet of the PM filter 40.

At 128, the control module 50 may determine whether the regenerationoperation has completed. For example, the regeneration operation may becompleted after the inlet temperature T_(in) is greater than or equal toa predetermined temperature threshold for a predetermined period. Iftrue, control may return to 104. If false, control may then return to108 where control of the desired amount of HC injection and the ratio ofauxiliary injection to post-combustion injection may continue (e.g., PIDcontrol) until the regeneration operation has completed.

The broad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification, and the following claims.

1. An engine control system, comprising: an injection determinationmodule that determines a desired amount of hydrocarbons (HC) to injectinto exhaust gas produced by an engine for regeneration of a particulatematter (PM) filter; and an injection distribution module that controls aratio of auxiliary injection to post-combustion injection based onengine load, engine speed, and the desired amount of HC injection,wherein auxiliary injection includes injecting HC into the exhaust gas,and wherein post-combustion injection includes injecting HC intocylinders of the engine during a period after combustion.
 2. The enginecontrol system of claim 1, wherein the injection distribution moduledetermines amounts of auxiliary injection and post-combustion injectionbased on the engine load, the engine speed, and the desired amount of HCinjection.
 3. The engine control system of claim 2, further comprising:a regeneration control module that generates control signals for anauxiliary HC injector and fuel injectors based on the amounts ofauxiliary injection and post-combustion injection, respectively.
 4. Theengine control system of claim 3, wherein the auxiliary HC injector islocated upstream from an oxidation catalyst (OC) in an exhaust treatmentsystem, and wherein the fuel injectors correspond to cylinders of theengine.
 5. The engine control system of claim 4, wherein theregeneration control module generates control signals for fuel injectorscorresponding to one bank of cylinders of the engine.
 6. The enginecontrol system of claim 2, wherein the injection distribution moduledetermines a factor using a lookup table that includes a plurality offactors relating to engine load and engine speed.
 7. The engine controlsystem of claim 6, wherein the injection distribution module determinesthe amount of auxiliary injection based on a product of the factor andthe desired amount of HC injection, and wherein the injectiondistribution module determines the amount of post-combustion injectionbased on a difference between the desired amount of HC injection and thedetermined amount of auxiliary injection.
 8. The engine control systemof claim 1, wherein the injection determination module determines thedesired amount of HC injection based on a temperature of the exhaust gasat an inlet of the PM filter.
 9. The engine control system of claim 8,wherein the injection determination module determines the desired amountof HC to inject into the exhaust gas based on at least one of exhaustgas flow and vehicle speed.
 10. The engine control system of claim 9,wherein the injection determination module performsproportional-integral-derivative (PID) control of the desired amount ofHC to inject into the exhaust gas.
 11. A method, comprising: determininga desired amount of hydrocarbons (HC) to inject into exhaust gasproduced by an engine for regeneration of a particulate matter (PM)filter; and controlling a ratio of auxiliary injection topost-combustion injection based on engine load, engine speed, and thedesired amount of HC injection, wherein auxiliary injection includesinjecting HC into the exhaust gas, and wherein post-combustion injectionincludes injecting HC into cylinders of the engine during a period aftercombustion.
 12. The method of claim 11, further comprising: determiningamounts of auxiliary injection and post-combustion injection based onthe engine load, the engine speed, and the desired amount of HCinjection.
 13. The method of claim 12, further comprising: generatingcontrol signals for an auxiliary HC injector and fuel injectors based onthe amounts of auxiliary injection and post-combustion injection,respectively.
 14. The method of claim 13, wherein the auxiliary HCinjector is located upstream from an oxidation catalyst (OC) in anexhaust treatment system, and wherein the fuel injectors correspond tocylinders of the engine.
 15. The method of claim 14, further comprising:generating control signals for fuel injectors corresponding to one bankof cylinders of the engine.
 16. The method of claim 12, furthercomprising: determining a factor using a lookup table that includes aplurality of factors relating to engine load and engine speed.
 17. Themethod of claim 16, further comprising: determining the amount ofauxiliary injection based on a product of the factor and the desiredamount of HC injection; and determining the amount of post-combustioninjection based on a difference between the desired amount of HCinjection and the determined amount of auxiliary injection.
 18. Themethod of claim 11, further comprising: determining the desired amountof HC injection based on a temperature of the exhaust gas at an inlet ofthe PM filter.
 19. The method of claim 18, further comprising:determining the desired amount of HC to inject into the exhaust gasbased on at least one of exhaust gas flow and vehicle speed.
 20. Themethod of claim 19, further comprising: performingproportional-integral-derivative (PID) control of the desired amount ofHC to inject into the exhaust gas.